1. Field of the Invention
This invention relates to digital computer bus systems. More specifically to an equal length symmetric bus topology that reduces the impact of transmission line reflections.
2. Art Background
A digital computer employs a computer bus to communicate information between the different parts of the computer system. Buses are simply a set of parallel electrical lines used to communicate information signals. FIG. 1 illustrates in block diagram form, a typical computer system with a bus 15. The bus 15 connects the processor 11 to memory 13 and to various Input/Output ("I/O") devices 12, such as displays, keyboards, etc.
There can be more than one bus in a single computer system. FIG. 1 illustrates a computer system with a main computer bus 15 and a secondary computer bus 17. The computer system uses the main computer bus 15 for high-speed memory access and other time critical functions. The secondary bus 17 operates at a slower speed in order to accommodate slower I/O devices 19. A bus controller 18 couples the main computer bus 15 to the secondary computer bus 17.
A typical microcomputer bus is physically constructed of a set of parallel traces embedded in a printed circuit board. Most microcomputers incorporate the computer bus as electrical traces etched on the main printed circuit board called the motherboard.
The clock speeds of modern computer processors are continually increasing in order to achieve faster performance. This is especially true for microprocessor based computer systems. As a result, the buses that the processors use to communicate information between the different parts of a computer system also need to function at increasingly higher frequencies. Computer designers, therefore need to carefully design computer buses to handle the higher operating frequencies in order to maintain high quality bus signals.
When the operating frequency of a computer bus, such as the bus 15 illustrated in FIG. 1, becomes very high, each of the bus signals in the bus 15 begin to exhibit transmission line effects which can degrade the quality of the bus signal. One type of transmission line effect that degrades signal quality is a transmission line reflection. Impedance discontinuities along the signal path of a bus signal cause transmission line reflections to occur. At each point in a signal path where an impedance change occurs, a transmission line reflection will be generated. The greater the impedance mismatch, the greater the transmission line reflection will be.
There are often a number of impedance discontinuities along the signal path of a computer bus signal. Common computer bus signal impedance discontinuities include mismatched loads, electrical connectors, line stubs, device input, and board layer changes. It is therefore desirable to design a computer bus that minimizes the effect transmission line reflections have on the signals.